At present, reduction projection-exposure methods that achieve high processing speeds are widely used in the manufacture of semiconductor integrated circuits. In recent years, with ongoing miniaturization of semiconductor integrated-circuit elements, projection lithography using soft X-rays having shorter wavelengths (0.5 to 50 nm) than conventional ultraviolet light, has been developed. Soft X-rays improve the resolving power of optical systems that otherwise are limited by diffraction of light, as described in Tichenor et al., Proceedings SPIE 2437:292 (1995). Soft X-ray lithography is also called EUV (extreme ultraviolet) lithography (abbreviated EUVL), which is the name most commonly used now. EUVL is expected to become the lithography technology of the future, offering resolving powers of 50 nm or less. Such resolution currently is not possible using conventional photolithography performed using wavelengths of approximately 190 nm or greater.
Because the index of refraction of materials is very close to one in the EUVL wavelength range, conventional optical elements used for refraction and reflection cannot be used. Consequently, grazing-incidence mirrors (providing total reflection due to their index of refraction being slightly less than 1) and multilayer-film reflectors (which combine and superpose multiple phases of light weakly reflected at layer interfaces) are used. The obtained reflectance is sufficiently high to be useful.
An EUV light source used in an EUVL apparatus radiates light of various wavelengths in addition to EUV wavelengths. Many of the wavelengths (e.g., ultraviolet light, visible light, infrared light, and the like having wavelengths longer than EUV wavelengths) are different from actual EUV light used for exposures. These non-exposure wavelengths are called OoB (out of band) light. If exposure light includes OoB light, the following problems generally occur:
(1) The OoB wavelengths expose the EUV optical systems to excess radiant energy. Absorption of this excess energy by reflectors of the projection-optical system causes the reflectors to exhibit thermal aberrations (e.g., aberrations caused by heat-deformation of the reflector). These aberrations deteriorate the performance of the projection-optical system.
(2) The resist on the wafer to be lithographically exposed have some sensitivity to OoB light. Hence, when certain wavelengths of OoB light (e.g., ultraviolet light and the like) reach the wafer, they produce background exposure “noise.” This noise does not help resolve the lithographic pattern on the wafer, has the same effect as flare in photolithography optical systems, reduces the contrast of the lithographic image, and actually deteriorates the resolving power of the EUV optical system.
(3) In addition to providing no beneficial contribution to the EUV lithographic exposure, OoB light reaching the wafer causes heating of the wafer, with consequent thermal expansion of the wafer. Thus, alignment precision of the wafer is degraded and distortion is increased.
Usually, a filter is used to block OoB light. An exemplary OoB-light-blocking filter suitable for use in the EUV-wavelength region is a free-standing film (membrane) type filter. The membrane filter is a very thin (1 micrometer or less) layer of beryllium (Be), zirconium (Zr), or the like, as described in U.S. Pat. No. 6,833,223. Unfortunately, such membrane-type filters are very fragile, are difficult to make in large diameters, and exhibit low transmission of EUV light (approximately 50% or less).
A multilayer-film reflector, which suppresses reflection of OoB light, has been proposed for use as an OoB filter in place of the conventional membrane-type filter in EUV optical systems, as described in Japan Kokai Patent Document No. Hei 6-148399. The multilayer-film reflector includes an antireflective layer, disposed on the topmost layer of the multilayer film, to prevent reflection of incident OoB light. Unfortunately, this type of reflector used as a filter not only absorbs more OoB light than a conventional multilayer-film reflector but also absorbs EUV light. Consequently, when using such a reflector as a filter, the intensity of EUV produced by the source must be correspondingly greater to have the same illuminance of EUV light on the wafer otherwise obtained when using a conventional thin-film filter. But, using the EUV projection-optical system with more intense EUV light causes more thermal deformation of the system, increases the aberrations of the system, and degrades the optical performance of the system.
Consequently, there is a need for projection-optical systems that provide reduced OoB radiation on the wafer and that exhibit less deterioration of their optical properties.